The present invention relates to an improvement of a gate drive circuit for a voltage-driven switching device, such as an FET or IGBT (insulating-gate bipolar transistor).
FIG. 16 shows an example of an IGBT used as a voltage-driven switching device, and FIG. 17 shows an example of an inverter main circuit used as a power converter.
In FIG. 16, reference numeral 1 designates an IGBT as a main device; 36 is a gate drive circuit power supply at a positive side for applying a voltage between the gate and emitter of the IGBT while it is being turned on; 37 is a gate drive circuit power supply at a negative side for applying a voltage between the gate and emitter of the IGBT while it is being turned off; and 38 is a control circuit for an inverter, which generates instructions to switch the IGBT on and off. Reference numeral 39 denotes an insulator, such as photocoupler (PC), for transmitting instructions from a low-electricity section to a high-electricity section to turn the IGBT on and off; 40 is a gate resistor for turning the IGBT on; 41 is a gate resistor for turning the IGBT off; 42 and 43 are switch transistors for connecting each gate resistor to the gate of the IGBT; and 44 is an amplifier for driving the transistors 42, 43. Signals transmitted via the insulator 39 cause a positive voltage or a negative voltage from the power supplies 36, 37 to be applied between the gate and emitter of the IGBT, to turn it on and off.
In FIG. 17, reference numeral 45 indicates a diode rectifier for converting AC to DC; 46 is a DC intermediate capacitor; 47 is an inverter comprising an IGBT and a diode (also simply referred to as "FWD") for converting DC to AC; and 49 is a snubber circuit for protecting the inverter from a spike voltage generated by an inductance 48 between the DC intermediate capacitor 46 and the inverter 47.
Factors which affect switching characteristics of the IGBT or gate drive circuit are: a gate resistance Rg; a gate drive circuit power supply voltage Vg; a gate current (current flowing in and out of the gate of the IGBT) Ig; a capacitance between the gate and emitter of the IGBT as seen from the gate drive circuit C.sub.GE ; and a capacitance between the gate and collector of the IGBT as seen from the gate drive circuit C.sub.GC.
Table 1 shows the direction of the change in each of the switching characteristics of the IGBT when the number of conditions (absolute value) of the IGBT or the gate drive circuit increase. When the number of conditions decrease, each switching characteristic of the IGBT changes in the direction opposite to that shown in Table 1.
TABLE 1 ______________________________________ Rg large Vg large Ig large C.sub.GE large C.sub.GC large ______________________________________ di/dt at turn on Low High High Low Low di/dt at turn off Low High High Low Low dv/dt at turn on Low High High Low Low dv/dt at turn off Low High High Low Low v.sub.peak at turn off Low High High Low Low i.sub.peak at turn on Low High High Low Low ______________________________________
On the other hand, if the inverter comprises a voltage-driven switching device, such as an IGBT, the wiring inductance present between the DC intermediate capacitor and the inverter causes a high voltage to the switching device, such as an IGBT or FWD, when the IGBT is switched on and off, as shown in Equation (1).
Equation 1
V.sub.CE =Ed+L.di/dt
V.sub.CE : Voltage applied to device PA1 Ed: DC intermediate capacitor voltage PA1 L: Wiring inductance PA1 di/dt: Current change rate during switching
Thus, when the inverter device is formed by using an IGBT and FWD, the device must have a voltage rating that satisfies Equation (1) or a snubber circuit must be added. Furthermore, it has been pointed out that a significant noise may be generated from the apparatus if dv/dt and/or di/dt is large during switching.
FIG. 18 shows approximate waveforms of the current and voltage when the device is being turned off, and FIG. 19 shows approximate waveforms of the current and voltage when the device is being turned on. As is apparent from these figures, when the device is being turned off, the magnitude of di/dt of the turn-off current of the IGBT affects the magnitude of the spike voltage directly applied to the IGBT by Equation (1). On the other hand, when the device is being turned on, the magnitude of di/dt of the turn-on current of the IGBT affects the magnitude of the spike current flowing through the IGBT (reverse recovery current flowing through the FWD on the opposite arm), which in turn affects the magnitude of di/dt during the reverse recovery of the FWD on the opposite arm. That is, when the di/dt at the time of turn-on of the IGBT increases, the di/dt at the reverse recovery of the FWD on the opposite arm also increases, which causes a large dv/dt to be applied to the FWD and also increases a spike voltage applied to it according to Equation (1).
Thus, the invention is aimed at eliminating a snubber circuit and reducing noise in a gate drive circuit for a voltage-driven switching device.